The blaze, first reported at 4:47 a.m., was about 80 percent contained by 3:30 p.m. No injuries were reported, no structures were threatened and no cause was announced.

The fire spewed a wall of smoke as it burned through brush and grassland but did not affect the forest canopy in its path along a drainage swale below the Mount Barnabe Fire Lookout.

The fire site was described as north of Sir Francis Drake Boulevard, south of Barnabe Ridge, east of Devil's Gulch and west of Barnabe Fire Road.

About 40 county firefighters were joined by 150 others from Cal Fire. They were equipped with nine engines, two water tenders and two bulldozers, an airplane dropping fire retardant, and a helicopter that dumped buckets of water scooped from Kent Lake. Crews from Novato, Ross Valley, San Rafael, Southern Marin and Tomales also responded to the fire.

County fire Battalion Chief Mike Giannini said fire crews had a tough time getting to and flanking the blaze a mile or so inside the park.

"It's a long narrow fire," Giannini said, adding state aircraft had to wait for fog to lift before they could lend a hand.

"This will be a ground battle," the chief said early on. "It will be a long day."

Winds were clocked at about 10 mph and humidity hovered at 90 percent.

Fire crews set up a command center at the Madrone group campground shortly after 5 a.m.

Trails and fire roads in the region were closed, but the park remained open.

County fire Battalion Chief Bill Roberts said the fire is expected to be fully extinguished by Friday — though the elements are not helping in the effort.

"We're making good progress," Roberts said around 1 p.m. Thursday, speaking from Madrone. "Wind was definitely a factor this morning. We're worried about a westerly wind coming up later this afternoon. The fuels are extremely dry."

Residents in San Geronimo Valley took the fire in stride as they talked about it at local grocery stores.

"A lot of people are worried, coming in asking about it," Hannah Anki, owner of Lagunitas Grocery and Delicatessen.

While the fire appears to be coming under control, those dry conditions — the result of Northern California's two-year drought — loom ominously over San Geronimo Valley residents, with the onset of fire season.

"I'm concerned," said Sandy Clark of Woodacre. "If there's going to be a fire, it's going to whip right through. One thing, though: we do have great firemen out here."